In a known mechanical spring-driven actuator, a cylindrical canister is interconnected with a plunger. A helical compression spring reacts between the canister and a structural member at the opposite end of the actuator. The spring biases the plunger to an extended position, and, correspondingly, resists retraction of the plunger. One or more components of the actuator support the spring internally and/or externally to maintain its shape without buckling.
The opposite ends of such actuators are typically connected to relatively movable components of a mechanical assembly to provide a biasing force. For example, such actuators can be used for counterbalancing. More specifically, such actuators have been used for biasing overwing emergency exit doors on aircraft by interconnection between the aircraft fuselage and hinge arms by which the exit doors are swingably mounted on the fuselage. The exit doors are normally latched in the closed position, with the actuator plungers retracted. When a door is unlatched, the spring force driving the actuator plungers is sufficient to swing the door open. Actuators for the same purpose may use tension or torsion springs, stretched or wound within the mechanism to provide the desired counterbalancing force or opening force.
In the known mechanical assemblies, external components are provided with sufficient backup structure to absorb and dissipate the inertial loads generated when the dynamic system is rapidly decelerated to a static condition, e.g., to limit the opening movement of an aircraft door. Also, these systems often require an additional "down lock" mechanism and backup structure to hold or lock the moving structure (such as the aircraft door) in the final counterbalanced (open) position. It is generally desirable that this down lock mechanism lock automatically when the travel stop is reached, and require a manual release operation in order to return the counterbalanced structure to its initial (closed) position. Depending on the geometry and the required loads involved, the down lock mechanism often is as heavy and complicated as the actuator, and the travel stop adds additional weight and complexity. Another problem with at least some unidirectional mechanical actuators is the risk of injury during installation or replacement, since the loads may be high and the spring's force must be overcome during installation in the mechanical assembly.